1. Field of the Invention
The present invention relates to a method of forming a semiconductor structure having at least a contact plug, and more particularly, to a method of forming a semiconductor structure having a gate structure and at least a contact plug, wherein a top surface of the contact plug is substantially higher than a top surface of the gate structure, and the top surface of the contact plug and a top surface of a cap layer formed on the gate structure are coplanar.
2. Description of the Prior Art
Poly-silicon is conventionally used as a gate electrode in semiconductor devices, such as the metal-oxide-semiconductor (MOS). However, with a trend toward scaling down the size of semiconductor devices, the conventional poly-silicon gate has faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of the gate dielectric layer, reduces gate capacitance, and worsens a driving force of the devices. Therefore, work function metals are used to replace the conventional poly-silicon gate to be the control electrode that is suitable for the high-k gate dielectric layer.
In conventional arts, after forming the transistor with a metal gate, a wiring system is formed thereon to electrically connect the metal gate and the source/drain regions, thereby providing signal input/output pathways for the transistor. The wiring system includes a plurality of contact plugs, and the double patterning technique (DPT) has been developed and taken as one of the most promising lithographic technologies to overcome the limitations of the conventional tools for forming complicated and intensive patterns. In other words, multiple photolithography processes are required to be performed during the conventional contact plug processes, therefore, the cleaning solution, the etchant or the chemical solvent used in the photolithography processes may directly contact the exposed metal gates, which may deform or damage the formed metal gates. Consequently, how to avoid the effects caused by the later processes and maintain the normal function of the metal gates is still an important issue in the field.